1. Field of the Invention
The present invention relates to a repeater station for relaying transmission signals with amplification and/or shaping of the transmission signals in a radio transmission system. More particularly, the present invention is concerned with the repeater station adapted to a digital multiplex radio transmission system wherein radio frame data constructed by inserting auxiliary signals for radio transmission into input frame data including multiple signals are transmitted and relayed.
2. Description of the Related Art
In a general digital multiplex radio transmission system, RFD (Radio Frame Data) are constructed in the sending end by inserting auxiliary signals into IFD (Input Frame Data) which are multiple signals conforming to a prescribed frame form. The RFD are modulated in a modulator and then transmitted from a transmitter. The auxiliary signals include a frame synchronization signal for frame synchronization of the RFD, an error-correcting code for detecting and correcting errors in radio transmission, and a signal of a digital service channel, etc. In the receiving end, the signal received in a receiver is demodulated in a demodulator and the auxiliary signals are removed to reproduce the IFD.
A required number of repeater stations is located between the sending end and the receiving end. In the repeater station, the received signal is demodulated into a baseband signal. The baseband signal is shaped by synchronizing same with a clock signal ticking time of the baseband signal and the shaped baseband signal is again modulated and then transmitted. If the auxiliary signal must be read/written in the repeater station, frame synchronization with regard to the RFD is performed to give access to the auxiliary signal.
As mentioned above, in the general digital multiplex radio transmission system, it is enough sufficient, for operation of the repeater station, that frame synchronization with regard to the RFD is afforded.
However, there is a special case where information included in the IFD must be accessible. The case is, for example, where signals conforming to a frame form prescribed in an NNI (Network Node Interface) in an ISDN (Integrated Services Digital Network) are transmitted. A data frame, conforming to an SDH (Synchronous Digital Hierarchy) as the NNI, includes an SOH (Section OverHead) area containing network management information. The network management information includes information for watching repeater stations. Accordingly, also in the repeater station, the SOH must be accessible, that is, the IFD must be accessible in the repeater station relaying the SDH frames.
Nevertheless, access to the IFD is not easy in the repeater station, because the RFD is generally constructed in asynchronism with the frame form of the IFD and therefore the IFD does not become accessible only by frame-synchronizing with the RFD.
This problem may be overcome if the RFD is constructed by inserting the auxiliary signals in synchronism with the frame form of the IFD. Then, the IFD becomes accessible only by frame-synchronizing with the RFD.
However, this solution raises another problem that a degree of freedom in design of the frame form of the RFD is lowered. If the ratio of the auxiliary bits, especially, the ratio of the error correcting code inserted to construct the RFD is higher, the capability of error correction is higher but the transmission rate must be elevated and therefore the bandwidth occupied in the radio frequency becomes large. For this reason, an appropriate frame form of the RFD must be designed in accordance with the quality of the radio transmission path by trading off both factors. Thus, the fact that the degree of freedom in design of the RFD is lowered is unfavorable.
The IFD may also become accessible in a construction where the auxiliary signals are removed after frame synchronization with the RFD; the transmission rate is altered using an elastic memory to obtain the IFD, and then frame synchronization with the IFD is performed, in the repeater station, as is usually done in the receiving end. But this construction is complex and requires large size circuits.